Add board presets, devices --clear, and test/UX fixes

Board presets:
- anvil new --board mega (uno, mega, nano, nano-old, leonardo, micro)
- anvil new --list-boards shows presets with compatible clones
- FQBN and baud rate flow into .anvil.toml via template variables
- Defaults to uno when --board is omitted

Devices --clear:
- anvil devices --clear deletes .anvil.local, reverts to auto-detect

templates/basic/_dot_anvil.toml.tmpl

@@ -3,10 +3,10 @@ name = "{{PROJECT_NAME}}"
 anvil_version = "{{ANVIL_VERSION}}"
 
 [build]
-fqbn = "arduino:avr:uno"
+fqbn = "{{FQBN}}"
 warnings = "more"
 include_dirs = ["lib/hal", "lib/app"]
 extra_flags = ["-Werror"]
 
 [monitor]
-baud = 115200
+baud = {{BAUD}}